In a broadband wireless communication system including a plurality of base stations, a mobile station (MS) may move towards a neighboring base station (BS) while maintaining a call state through a handover. When each base station deals with two or more frequency allocations, the mobile station performs a handover within a frequency allocation (FA) channel (hereinafter, referred to “intra-FA handover”) or a handover between frequency allocation channels (hereinafter, referred to as “inter-FA handover”). The intra-FA handover and the inter-FA handover will be described below with reference to FIG. 1.
FIG. 1 illustrates a conventional intra-FA handover and inter-FA handover performed by a mobile station (MS).
Referring to FIG. 1, the mobile station, which receives a service and to which an FA2 is allocated among n FAs used by a BS1 100, performs a handover to a BS2 110 which uses n FAs. When the mobile station, which uses the FA2 allocated by the BS1 100, performs a handover by using an FA2 newly allocated by the BS2 110, it is called an intra-FA handover. When the mobile station, which uses the FA2 allocated by the BS1 100, performs a handover by using one FA newly allocated from the FAs of the BS2 other than the FA2, it is called an inter-FA handover.
When performing the intra-FA handover, a scanning operation is not additionally required because the signal strength of a neighboring BS can be recognized by receiving a preamble of the neighboring BS. However, the scanning operation is performed when the MS performs the inter-FA handover. When the inter-FA handover is performed, the MS has to release an FA currently allocated and used and then has to scan another FA of the neighboring BS. Thus, at this time, service disconnection is inevitable.
Now, a process in which BS signals are scanned by an MS in an Institute of Electrical and Electronics Engineers (IEEE) 802.16e system as an example of a broadband wireless communication system will be described with reference to FIG. 2.
FIG. 2 illustrates a process of scanning BS signals in a convention IEEE 802.16e system.
Referring to FIG. 2, a BS 210 periodically broadcasts a MOB-NBR_ADV message to a mobile station (MS) 220 in step 202. The MOB_NBR_ADV message includes information on a neighboring BS.
Examples of important parameters of the MOB_NBR_ADV message include a management message type that represents a type of message to be transmitted, a configuration change count that represents the number of times of changing configurations, N_NEIGHBORS that represents the number of neighboring BSs, Neighbor BS-ID that represents Identifiers (IDs) of the neighboring BSs, and extra neighbor information (e.g., Type-Length-Value (TLV) encoded neighbor information) that represents other information related to the neighboring BSs. Representative information included in the TLV is a physical frequency that indicates a physical channel frequency of the neighboring BSs.
In step 204, if the neighboring BS has to be subject to a scanning operation or if a target BS has to be determined for a handover operation, the MS 200 transmits a MOB_SCN_REQ message to the BS 210 and thus requests a scanning interval time. The MS 200 does not have to specify a BS_Index or a BS ID to be scanned among parameters of the MOB_SCN_REQ message. Therefore, the MS 200 may transmit only a part of the MOB_SCN_REQ message. A format of the MOB_SCN_REQ message will be described below in detail with reference to FIG. 3.
Upon receiving the MOB_SCN_REQ message, in step 206, the BS 210 determines a scanning start time of the MS 200 and transmits to the MS 200 a MOB_SCN_RSP message that informs whether to scanning is allowed or not. When the MS 200 transmits a simplified MOB_SCN-REQ message, only a part of the MOB_SCN_RSP message is transmitted by the BS 210. The BS 210 may transmit the MOB_SCN_RSP message without having to receive a request of the MS 200. In this case, a BS_Index parameter or a BS ID parameter has to be specified. A format of the MOB_SCN_RSP message will be described below in detail with reference to FIGS. 4A and 4B.
After receiving the MOB_SCN_RSP message, in step 220, the MS 200 performs scanning on the basis of all pieces of FA index information included in the MOB_NBR_ADV message received in step 202.
FIG. 3 illustrates a format of a MOB_SCN-REQ message in a conventional IEEE 802.16e system.
Referring to FIG. 3, the MOB_SCN-REQ message is transmitted to a BS so that an MS requests a scanning interval time in order to search for an available BS or to determine a target BS to be handed over.
FIGS. 4A and 4B illustrate a format of a MOB_SCN-RSP message in a conventional IEEE 802.16e system.
Referring to FIGS. 4A and 4B, the MOB_SCN-RSP message is transmitted by a BS in response to a MOB_SCN-REQ message transmitted by an MS. That is, the BS transmits to the MS the MOB_SCN-RSP message including FA information so as to allow scan-reporting to be started upon receiving a scanning request.
Conventionally, when the BS broadcasts a MOB_NBR-ADV message including a plurality of FA information pieces, the MS cannot recognize a channel state of another FA. Thus, an FA of a neighboring BS is scanned by transmitting the MOB_SNC-REQ message for all specified FAs. However, it is not effective for the MS to scan all FAs specified in the MOB_NBR-ADV message. This is because a service is disconnected in an FA scan time, and a time corresponding to 10 frames (or 50 msec) is required to scan one FA. If the MS scans 3 FAs, service disconnection occurs for 150 msec or more.
Therefore, there is a need for an apparatus and method for scanning only a specific FA by receiving FA information to be scanned by the mobile station.